This invention relates to aircraft navigation in general and more particularly to a digital TACAN system utilizing a stand-alone digital processor.
TACAN equipment is used on most military aircraft to provide an indication of the distance and bearing to any selected TACAN station. In a TACAN system a ground station transmits TACAN pulses at a rate of approximately 2700 pulses per second. The ground station antenna rotates 15 times per second and has an antenna directivity pattern which has a cardioid component to it. This causes the pulses received by the aircraft to have a 15 hertz modulation of plus or minus 20 percent amplitude and a phase which depends upon the aircraft's bearing with respect to the TACAN station. The transmitter also sends a reference pulse code once each antenna cycle, at a fixed point in the antenna rotation, to establish an absolute reference phase. The aircraft equipment determines the bearing by comparing the phase of the amplitude modulated signal with the reference phase pulses. For further accuracy, a ninth harmonic is also superimposed on the antenna directivity pattern producing a 135 hertz amplitude modulation. This will be present at the receiver at phase coherence with the 15 hertz modulation described above and permits making a finer determination of bearing in a manner similar to that used to older two-speed servo systems.
The most relevant prior art known to the inventors is the digital TACAN processor disclosed in U.S. Pat. No. 3,940,763 having the same assignee as the present application. The referenced patent discloses a signal processor which can be implemented using a general purpose computer or special purpose digital hard wire system to perform only the TACAN processing function. The system of the patent employs the technique of a least square recursive (Kalman) filter to update the current bearing estimates with each new data input. In performing the bearing computation, a Kalman filter provides a continuous estimate of the reference frequency .omega. and the reference phase .theta.. A second Kalman filter acting as a bearing signal tracker accepts amplitude data, data from the first filter and, also, the time of arrival of each bearing pulse and updates five-state variables related to the amplitude and phase of the 15 and 135 hertz modulations and a DC amplitude component which gives the average amplitude of the modulated bearing pulses. A bearing computer uses the outputs of the bearing signal tracker and the reference phase tracker to compute the final bearing to be displayed to a pilot.
Although the system of the described prior art patent operates satisfactorily, the throughput demands made on the computing hardware to process all incoming TACAN pulses are high and result in objectionable hardware complexity and cost. In practice, it has been found desirable to reduce the number of TACAN pulses processed to minimize hardware complexity with the attendant compromise in performance.